Splanchnic utilization of enteral alanine in humans

The splanchnic bed extracts the majority of the enteral nonessential amino acids glutamine and glutamate, while extracting a much smaller proportion of essential amino acids such as leucine and phenylalanine. Alanine is an abundant nonessential amino acid that plays an important role in hepatic gluconeogenesis and ureagenesis. However, its enteral fate has not been studied. Twelve normal healthy postabsorptive adults received a 7-hour infusion of [1-C-13]alanine, 3.5 hours intravenously (IV) and 3.5 hours via a nasogastric tube (NG). The order of infusion was randomized among subjects. Alanine kinetics were calculated from the enrichments of plasma alanine C-13 and expired (CO2)-C-13. The alanine appearance rate (R-a), measured during the IV tracer infusion, was 279 +/- 17 mu mol/kg/h; 92% +/- 2% of the IV-infused and 86% +/- 2% of the NO-infused [1-C-13]alanine tracer was recovered as (CO2)-C-13. From the difference in plasma alanine C-13 enrichment between IV-infused and NG-infused tracers, we determined that the splanchnic bed extracted 69% +/- 1% of the enterally delivered alanine tracer on the first pass during absorption. Only one third of the enteral alanine passed intact through the splanchnic bed and was made available to systemic tissues. Of the enteral alanine extracted, 83% +/- 3% of the carboxyl-carbon label was recovered as CO2, leaving only 17% of the sequestered alanine available for use in splanchnic protein synthesis. Thus, the splanchnic bed, presumably the liver, extracts and metabolizes most of the enterally delivered alanine. Copyright (C) 1999 by W.B. Saunders Company.